Photographic elements made predominantly of silver chloride, with minor amounts of silver bromide and iodide (e.g., &gt;70 mole % chloride), are well known in the prior art. One major advantage of silver chloride over other photographically-useful silver halides is that it possesses greater aqueous solubility and thus allows for more rapid processing of exposed elements. However, since silver chloride-containing elements exhibit in general lower photographic speed than those containing mainly silver bromide, the use of such elements has been limited to graphic arts applications (e.g., contact, low-speed camera films, etc.). It would be desirable to use the significant benefit of rapid processibility in many of the silver halide art fields where chloride-rich emulsions are not already commonly employed because of photographic speed limitations.
It is also well known in the art that silver chloride strongly favors the formation of cubic crystals having {100} crystal faces. In the majority of photographic emulsions, silver chloride crystals when present are in the form of cubic grains. With some difficulty it has been possible to modify the crystal habit of silver chloride. Claes et al., "Crystal Habit Modification of AgCl by Impurities Determining the Solvation", The Journal of Photographic Science, Vol. 21, pp. 39-50, 1973, teaches the formation of silver chloride crystals with (110) (rhombic dodecahedral) and {111} (octahedral) faces through the use of various grain growth modifiers. Wyrsch, in "Sulfur Sensitization of Monosized Silver Chloride Emulsions with {111}, {110}, and {100} Crystal Habit", Paper III-13, International Congress of Photographic Science, pp. 122-124, 1978, discloses a triple-jet process in which silver chloride emulsions characterized by grains of modified growth habit are precipitated in the presence of ammonia and small amounts of divalent cadmium ions.
Tabular grain silver halide products are also known in the prior art and present the user with some considerable advantages over conventional grain products, e.g., those products having semi-spheroidal grains. The tabular products exhibit higher covering power, improved sharpness, can be more effectively spectrally sensitized, are more easily developed and can tolerate a higher level of hardening without loss in covering power, each providing quite an advantage over conventional grains.
There are now several methods available for making tabular grain high-chloride emulsions. Wey U.S. Pat. No. 4,399,215 discloses the use of ammonia under prescribed pH and pAg conditions to produce large, thick tabular chloride grains. Wey and Wilgus U.S. Pat. No. 4,414,306 discloses a process for tabular grain silver chlorobromide growth which relies on precise control of the molar ratio between chloride and bromide ions; however, the chloride content of the emulsion grains so produced is limited to no more than 40 mole percent. In Maskasky U.S. Pat. No. 4,400,463, high-aspect-ratio tabular grain formation is carried out in the presence of a growth modifying amount of an aminoazaindene and a synthetic peptizer having a thioether linkage used in place of gelatin. Maskasky U.S. Pat. No. 4,713,323 describes a process for precipitation of tabular grain high-chloride emulsions using a dispersing medium composed of a gelatino-peptizer having no more than 30 micromoles of methionine per gram and at least a 0.5 molar concentration of chloride ion. In Takada et al. U.S. Pat. No. 4,783,398, a large class of sulfur-containing heterocyclic compounds are disclosed as growth modifying agents for the precipitation of tabular high-chloride emulsion grains.
Two recent U.S. Pat. Nos. Tufano 4,801,523 and Tufano and Chan 4,804,621, describe processes for the precipitation of {111} octahedral and tabular grain chloride-rich emulsions. A specific class of aminoazapyridine growth modifiers is used to produce well-formed, noncubic emulsion microcrystals in a conventional gelatin growth medium. Since these growth modifiers exhibit acid-base behavior over a pH range commonly employed for silver halide precipitation (e.g., pH 2.5-9), the pH of the growth medium is important to obtain the desired grain morphology. It has now been discovered that the emulsion grains produced by these processes show signs of morphological instability during typical emulsion preparation steps after precipitation in the absence of the grain growth modifier. That is, under emulsion conditions where the surface-adsorbed growth modifier may desorb, e.g., pH &lt;2.5, and wash free of the emulsion grains, chloride-rich {111} octahedral and tabular grains may, through a ripening process, deform and revert to the more thermodynamically stable {100} cubic form. Thus if unchecked, much of the advantages of the noncubic high-chloride grains, i.e., unique sensitizability of the {111} high-chloride surface, high covering power, etc., are lost in the final photographic element itself. Therefore there is a need to more effectively stabilize noncubic emulsion grains during preparation steps subsequent to precipitation, wherein the grains are formed in the presence of a pH-sensitive grain growth modifier, and at least 50 mole percent of the grains of the emulsion are chloride to provide photographically useful emulsions.
An object of this invention is to provide a method to morphologically stabilize {111} noncubic chloride-rich grain types so as to prevent ripening to the {100} cubic form.
Another object of this invention is to provide a method to morphologically stabilize {111} noncubic emulsion grains without requiring the presence of a grain growth modifier during emulsion preparation steps subsequent to precipitation thus providing an emulsion having greater sensitization latitude.
A further object of the invention is to provide a method for introducing a shell onto a chloride-rich silver halide grain core without compromising the well-defined {111} surface structure or resulting in epitaxial deposition, or re-seeding.
Still another object of the invention is to provide a silver halide photographic emulsion with the combined advantages of the rapid processibility of chloride-rich grains and the spectral and chemical sensitizability of bromide-rich grains with little or no disadvantages of either.